Environmental concerns and regulations have spurred the need to reduce the amount of hazardous hydrocarbon vapors emitted by vehicles and have led to the development of new technologies for manufacturing plastic fuel tanks. A fuel tank constructed from a single layer of a plastic, such as high density polyethylene, has been found to have an unacceptably high permeation rate of fuel vapor therethrough. Therefore, current plastic fuel tanks are formed with multiple layers one of which is a vapor barrier layer and these tanks have reduced the amount of hydrocarbons released into the atmosphere by as much as 60 times over single layer untreated plastic fuel tanks.
A typical multi-layerlayer plastic fuel tank construction comprises an outer layer of high density polyethylene, an inner layer of high density polyethylene, and a vapor barrier layer disposed between them. The vapor barrier layer is typically a polymer such as ethylene vinyl alcohol which requires an adhesive layer adjacent both the outer and inner layers to join the high density polyethylene with the ethylene vinyl alcohol. A multi-layerlayer plastic fuel tank is substantially more difficult to manufacture than a single layer plastic fuel tank and is usually made by a multi-layer extrusion and blow molding process.
Previously, a separate monolayer cap, or a separate multiple layer cap formed by a separate molding process, such as injection molding, has been provided to cover each opening into the fuel tank. Undesirably, the monolayer cap permits fuel vapor to permeate therethrough and the separate molding process to provide a multiple layer cap is relatively costly and time consuming to perform.